When a presently conventional Graygate gate valve of Gray Tool Company, Houston, Tex., U.S.A. is operating in an open position under pressure and then closed, the pressure is trapped within the valve cavity. With the valve closed, this internal pressure acts against the gate segments forcing said segments against the valve seats. This same pressure also acts against the cross-sectional area of the stem. Thus, the area of the stem where it passes through the stem packing must be of such area to cause the internal pressure to generate sufficient upward force to overcome the frictional forces resisting stem movement developed by the internal pressure between the gates and seats, thereby allowing the stem to rise and engage the backseat.
In a conventional safety valve of the gate valve type, such as shown in McGee, U.S. Pat. No. 3,789,875, issued Feb. 5, 1974, the stem diameter is made large enough to overcome the frictional forces thereby insuring that the stem will always move. The valve development of the present invention is most closely related to the valve developments shown in McGee et al, U.S. Pat. No. 4,149,558, and in McGee commonly assigned copending U.S. Pat. application Ser. No. 955,232, filed Oct. 27, 1978.